The two main types of alternating current motors used in the world are the induction motor and the permanent magnet synchronous motor (PMSM). Also used in much lower quantities are the wound rotor synchronous motor (for high power), the synchronous reluctance motor (for very rugged environments) and the switched reluctance motor (low cost but poor performance due to mechanical vibrations).
To speed control the induction motor, all that is needed is to supply alternating currents to the windings and vary their frequency to vary the motor speed. For all the other types of motors, a rotor position sensor is normally also used to synchronise the phase of each winding current to the rotor angle so that the applied alternating current has the correct phase to produce a positive rotor torque.
The problem with this sensor is that it costs money (for small motors it can cost more than the motor itself), it puts restrictions on the motor's operating environment (the sensor is much less rugged than the motor), it reduces the reliability of the motor, and the extra leads to connect the sensor to the controller must be accommodated.
Over at least the last 20 years there has been a large volume of research effort directed at developing control methods to eliminate the sensors, especially for the most used motor, the PMSM. However even though the research effort has been intense for a long time, only limited commercial success has been achieved.
There are several simple sensorless control schemes for PMSM's that have wide commercial use. One is the stepper motor, which is widely used in printers and photocopiers. This is a low speed motor which is controlled by injecting a high current into one winding at a time which forces the rotor's magnetic field and thus the rotor itself to align with the field generated by the current. The high current is moved sequentially from one winding to the next dragging the rotor with it. It is also possible to hold the rotor between two steps by putting partial currents in two adjacent windings, a technique called microstepping. These motors have a high number of poles so that one step results in the rotor moving only one or two degrees. This method does not work at higher speeds due to instabilities encountered when the speed matches a mechanical resonance and due to distortion of the current waveforms from the rising back EMF voltage.
There is also another simple sensorless control technique which has found wide application but which only works at high speed. It is used in hard disk drive motors and other small, high speed motors. This technique consists of supplying current to two of the three motor windings at any one time and using the disconnected winding to measure the PMSM's back EMF to determine the position of the rotor. This position information is then used to determine when to rotate the currents to the next set of windings. This method is not selfstarting due to the lack of back EMF at low speed, so the motor is first started as a stepper motor then changed over to back EMF control when the speed is increased to a sufficient level.
For applications requiring high torque at zero speed, fast speed reversal, or the application of smooth sinusoidal currents for smooth torque output, the above methods do not work. Research in the field has concentrated on finding sensorless methods which are suitable for these applications. The only sensorless controller for these applications that has gained commercial use is a combination of two techniques one for high speed operation and one for zero and low speed operation.
The technique used for high speed running is to determine the rotor position from the back EMF as before, but to derive the back EMF by measuring the motor terminal voltages and subtracting the voltage drop across the motor winding resistance and inductance, leaving the back EMF voltage as the remainder. There are two obstacles to using this method, particularly at very low speeds. First, it is very difficult to sense and filter the motor terminal voltages and currents to extract the rotor speed and position with minimal delay time at the low end of the speed range and second, the motor resistance changes with motor temperature making it very difficult to obtain accurate values for it. Most recent research in this type of sensorless control has centred on solving these two problems.
The technique used for low speed running is to determine the rotor position by measuring changes in winding inductance, usually by injecting a high frequency test current into the windings while the motor is running. Unfortunately, this requires a specially designed motor in which the winding inductance varies in a predictable way with rotor position. Many researches are beginning to accept that there is no alternative to this compromise. For many applications, including direct drive washing machine motors, the use of motors of this type is not possible, due to their higher cost and design restrictions.
Throughout this specification the term ‘low speed’ refers to motor speeds low enough so that damping of the natural resonance of the motor is affected mainly by the motor resistance. These motor speeds are typically considered to be speeds below half the natural resonant frequency as measured on the rotor shaft. The term ‘high speed’ refers to motor speeds high enough that damping of the natural resonance of the motor is affected mainly by the change in motor speed with load torque and where changes in motor resistance have little effect. These motor speeds are typically considered to be speeds above twice the natural resonant frequency as measured on the rotor shaft. The transition between the low speed and the high speed region is gradual with low and high speed effects not completely eliminated at any speed. The natural resonance of the motor is the resonance resulting from the interaction between the restoring torque produced by the misalignment of the back EMF with the applied rotating voltage vector and the shaft inertia. The frequency of this resonance for a two pole motor is the natural resonant frequency ωn referred to in the document.
The terms ‘low frequency’ and ‘high frequency’ refer to the electrical frequencies of the motor winding voltages and currents corresponding to ‘low speed’ and ‘high speed’ operation where the frequency is the speed times the number of pole pairs in the motor. In the description of the AC motor controllers and in the example simulations of AC motor controllers in this document, it is assumed the motor has only one pole pair to simplify the explanation of the operation of these controllers without affecting the general applicability of the method. For the case of the single pole pair motor, the frequency and the speed are the same. However the skilled addressee will readily appreciate how to extend the method to the case of motors having multiple pole pairs.